1. Field of the Invention
The present invention relates to a toner for developing electrostatic latent images, and more specifically relates to a toner for developing electrostatic latent images fused by heat and pressure for use in electrophotographic apparatus of a digital type.
2. Description of the Related Art
Conventional electrophotographic apparatus in general use are typically electrophotographic apparatus of an analog type for copiers and the like which expose a document with light from a light source and form an electrostatic latent image on a photosensitive member by means of the light reflected from said exposed photosensitive member. Electrophotographic apparatus of a digital type which develop images by supplying a developer including a toner to an electrostatic latent image formed by dot units obtained by digital writing methods have been realized for facsimiles of the electrophotographic type, digital copiers which form images based on image information read by an image reader, or printers using the output from a computer terminal.
In electrophotographic apparatus of a digital type, an electrostatic latent image is formed in dot units on the surface of a photosensitive member by digital writing thereon via exposure by a light beam, said electrostatic latent image is developed standard development or reversal development using a toner, transferring the obtained toner image onto a recording medium such as paper or the like, and fusing the transferred toner image onto the recording medium to produce the recorded image. Toners used in the aforesaid digital methods are required to have minimal running together of dots caused by toner breakdown during transfer and fixing, i.e., excellent dot reproducibility. When dot reproducibility is poor, low density halftone reproducibility is lost and image breakdown results, causing reduced image quality.
On the other hand, toner fixing methods used in conventional electrophotographic apparatus typically use heat and pressure fixing methods providing an arrangement of a heating roller having a heat-resistant elastic layer of silicon or the like or a heat-resistant separation layer of polytetrafluoroethylene (PTFE) superimposed over the surface of a metal core provided with an internal heating means, and a pressure roller provided with a heat-resistant layer on the surface thereof which makes pressure contact with said heating roller, wherein a toner image is fixed when a transfer medium bearing a toner image on the surface thereof passes between said rollers.
In the aforesaid type of heating and pressure fixing method, during fixing a portion of the toner is transferred to the fixing roller through contact of the toner with the pressure roller or like fixing roller or film member while in a heat fusion state, said portion of toner thereafter being again transferred to a subsequent transfer member and causing an offset phenomenon. Furthermore, dot reproducibility may be reduced due to pressure-induced toner deformation. Thus, toners used in heat and pressure fixing methods are required to have excellent offset resistance and dot reproducibility.
The previously described electrophotographic apparatus are available in a wide variety of types from low speed to high speed, or low to high fixing temperature apparatus. Since the speed with which the toner passes through the heating roller fixing device differs depending on the speed of the system, the toner is required to have a range of fixing efficiency, so as to be normally suitable for a specific apparatus type. However, using a different toner for each apparatus type in disadvantageous from the perspective of cost of manufacturing the toners, and it is therefore desirable to provide a toner that can be used for a broad range of apparatus regardless of system speed or fixing temperature.